The present invention relates to a powder coating composition. More specifically, the invention relates to a powder coating composition which comprises a mixture of
(A) a polyester resin containing as a functional group a hydroxyl group.
(B) a polyacyl-(N-lactam) compound or polyacyl-(N-imide) compound and
(C) a solid epoxy resin.
In general, the properties required of a powder coating composition are (1) the necessity to be solid and so brittle as to permit easily pulverization at the normal temperature, (2) the possession of the free fluidity as powder at a temperature below 50.degree. C. without blocking, (3) the possession of a good melt flow of resin necessary for forming a coating film on a flat surface and a proper gelling time at the time of baking the coating film at a temperature above 130.degree. C., (4) the capacity of being kneaded with an auxiliary substance such as a pigment, a flowing agent or an antistatic agent at a temperature of about 100.degree. C. without undergoing premature crosslinking, and (5) the possession of sufficient crosslinking density and a sufficient glass transition temperatures after baking. It is apparent that not only the inherent properties of the basic resin but also the selection of a curing agent are key points for satisfying the above mentioned various requirements.
Hitherto, epoxy resins are known to be useful as resins for powder coating composition. However, epoxy resins possess poor weather-resistance and are thus unsuited for applications subject to outdoor exposure for a long period of time. Known as resins possessing excellent weather-resistance are acrylic resins using a melamine resin etherified by alcohols as curing agent which, however, are poor in stability on storage and permit blocking at elevated temperatures; for example during the summer season.
There is also known to cure a carboxyl group-containing acrylic resin or a polyester resin using as a curing agent an epoxy resin such as triglycidyl isocyanurate. However, these resin compositions are also poor in stability on storage and tend to be subject to blocking at elevated temperature. In addition, the quality of the coating film prepared from these resin compositions cannot be said to be satisfactory.
As the result of many studies made to overcome the above mentioned drawbacks, it has now been found that a composition consisting essentially of a specific polyester resin, a curing agent of polyacyl-(N-lactam) compound and a solid epoxy resin are free of these drawbacks. The present invention is based on the above finding.